Some fuel injection systems have been designed as unit injectors which incorporate an hydraulically driven pressure intensifier with a stepped plunger for injecting fuel into an engine's cylinder where the fuel delivery and timing are controlled by an electronically controlled valve. The spray pattern of each injector is controlled by means of modulating the base oil pressure supplied to each unit injector.
It is known that in many diesel engines the optimum injection curve shapes vary depending on the engine's operating conditions. A pilot injection of a small amount of fuel separate from a main injection may be required at some operating conditions and a boot-shaped injection at other conditions, or a sharp leading front of an injection curve may be the best for another engine speed and load. The correlations between engine operating conditions and the optimum shapes of the injection curves are often complex. Therefore it is desirable for a diesel injection system to have the shape of an injection curve electronically controlled, so that an engine management system can set the optimum injection characteristics for a wider range of engine operating conditions.
Known unit injection system do not enable control of an injection curve shape independently from the actuating pressure due to the lack of a control channel which can be connected to a nozzle's locking chamber during certain stages of a plunger's metering and injection strokes.
The present invention concerns hydraulically actuated electronically controlled unit injection (HEUI) systems which are well known. The closest art known to the present invention is that of PCT/AU98/0073, the contents of which are incorporated herein by reference.
The difference between the injector and injection system of a first aspect of the present invention and that disclosed in PCT/AU98/0073 resides in the provision of an external groove on the plunger for connection of a plunger's compression chamber with a nozzle's locking chamber during an injection cut-off period.
A second aspect of the present invention resides in the inclusion of a control channel for stabilization and control of the pressure in the locking chamber during parts of the metering and injection strokes of a pressure intensifier, wherein this control channel and the locking chamber can be disconnected from each other by the plunger during an injection cut-off. The pressure in the control channel is typically controlled by an engine management system. When the control channel pressure is increased, the pressure in the compression chamber required to open the nozzle and begin the injection also increases, therefore the leading edge of the injection curve steepens. By means of varying the pressure in the control channel the shape of the leading edge of an injection curve can be controlled.
It is preferable to join the control channels of a set of unit injectors of an engine into a common control chamber with pressure in this chamber controlled by an engine management system. This ensures uniform injection timingmmon control chamber with pressure in this chamber controlled by an engine management system. This ensures uniform injection timing and shape of injection curves in the engine cylinders, simplifies the injection system design and helps keep the cost down as in this case only one pressure regulator is required and it can be mounted anywhere on an engine.
Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or integer or group of elements or integers but not the exclusion of any other element or integer or group of elements or integers.